1. Field of the Invention
The present invention relates to a developing method for visualizing an electrostatic latent image and more specifically to a developing method using a single-component developer capable of forming a dot copy of excellent tone gradation and a line copy of excellent sharpness and of high fidelity reproduction of solid blacks.
2. Description of the Prior Art
According to the electrophotography disclosed in U.S. Pat. No. 2,297,791 of Carlson, a photosensitive body having a photoconductive layer is charged uniformly, an electrostatic latent image is formed over the photosensitive body by an image exposure, the electrostatic latent image is visualized, i.e., developed, by using a substance such as a charged powder, and then the visualized image is transferred and fixed to a sheet. The cascade developing process, the magnetic bursh developing process and the liquid developing process are well-known processes applicable to developing the electrostatic latent image in the above-mentioned electrophotography. U.S. Pat. No. 2,895,847 discloses another important developing method, namely, a transfer developing method using a toner carrying member called as "a donor". The transfer developing method disclosed in the above patent is the general designation of (1) a developing method in which the toner layer and the photosensitive body are disposed with a gap therebetween and the toner flies across the gap, (2) a developing method in which the toner layer and the photosensitive body are disposed in rotary contact with each other and (3) a developing method in which the toner layer and the photosensitive body are disposed in sliding contact with each other, and this transfer developing method is well-known also as "the touch down developing method".
However, the transfer developing method has a serious problem, a problem of fogging in the background. U.S. Pat. No. 2,839,400 proposed a noncontact transfer developing method to improve the fogging. However, to make a toner fly across the gap between a photosensitive body and a donor as proposed in said patent, the width of the gap needs to be 0.05 mm or less or, if possible 0.03 mm or less, which is quite difficult in respect of the mechanical accuracy of the photosensitive body and the donor. In order to solve such a problem, U.S. Pat. Nos. 3,866,574, 3,890,929 and 3,893,418 disclosed methods in which an alternating electric field is formed between the photosensitive body and the donor. In U.S. Pat. No. 3,866,574 in particular, the relation between the developing gap and the alternating electric field is described. According to the same U.S. Patent, the conditions of the developing gap D.sub.g, the amplitude of the alternating electric field V.sub.p-p and the frequency f of the alternating electric field for the most satisfactory line development and the least fogging in the background are those meeting: 0.05 mm.ltoreq.D.sub.g .ltoreq.0.18 mm, 1.5 kHz.ltoreq.f.ltoreq.10 kHz and V.sub.p-p .ltoreq.800 V.
As regards the charge of the toner, even if the toner is manufactured and prepared in accordance with a fixed recipe, the charges of toner particles distributes in a distribution due to the dispersion of the particle size and the dispersion of the individual toner particles in physical properties, however, the charges distribute about a fixed value within a narrow range. Accordingly, the noncontact transfer developing method disclosed in U.S. Pat. No. 3,866,574 has a threshold for the toner to fly across the developing gap (designated as "flying threshold" hereinafter) in surface potential, and hence toner adheres to a surface of a surface potential greater than the flying threshold, whereas toner does not adhere to a surface of a surface potential smaller than the flying threshold. Thus, this noncontact transfer development method has a critical developing characteristic and produces a copy of extremely high .gamma. (.gamma.=the gradient of a characteristic curve of copy density relative to the potential of electrostatic image) and insufficient tone gradation. Furthermore, even if the charges of toner particles distributes over a wide range, only some of the toner particles can fly when the amplitude V.sub.p-p of the alternating electric field is 800 V or below and a copy of high .gamma. value results.
Japanese Patent publication No. Sho 58-32375 discloses a developing method to improve the critical developing characteristics dominated by the flying threshold restricting the flying of toner, namely developing characteristic represented by a large value of .gamma. and incapable of developing a latent image of satisfactory tone gradation. The invention disclosed in the Japanese patent publication is characterized by applying an alternating electric field to the developing gap in order to cause the alternate repetition of the transfer of the toner from a toner carrying body to a photosensitive body and the reverse transfer of the toner from the photosensitive body to the toner carrying body. In the specification of said patent, it is described that the alternate repetition of the transfer and the reverse transfer of the toner is scarecely effective when the frequency of the applied vias voltage is 2 kHz or above and is quite effective when 1 kHz or below.
This developing method in which a low-frequency alternating electric field is applied to the developing gap, is deemed to be effective in respect of causing the faithful adherence of the toner according to the surface potential when the charges of toner particles distribute within a narrow range and the flying of the toner across the developing gap is restricted by a definite threshold.
However, in case of the noncontact transfer developing process, if the electrostatic latent image has a high frequency, lines of electric force are not decomposed on the toner carrying body when the developing gap is 0.1 mm or above, and thereby the same electric field is formed over the image section and the nonimage section, that is, an image or picture formed by extremely fine lines or dots is blurred and becomes indistinct. This phenomenon will be described in detail hereunder. In the description, the degree of "indistinctness" is represented by an index M defined by a formula EQU M=(1-10.sup.-.DELTA.D)/(1+10.sup.-.DELTA.D)
where .DELTA.D is the density difference between the image section and the nonimage section. FIG. 1 is a graph showing the dependence of M on space frequency. From this, it is known that picture elements can be resolved to a density of about 5 l.p(line pair)/mm, but when the density is 6 l.p/mm or greater the image section and the nonimage section cannot be discriminated at all. Furtheremore, it was found through the microscopic observation of a copy image that the reduction of M was attributable to the blur of the developed image.
On the other hand, this method has the developing characteristics for dot-copies as shown in FIG. 2. When the line density is 65 line/in. or above., the image section becomes indistinct and the range of the developed image disagrees with the range of the image input. Consequently, the developed image of a dot-copy of high line density generally becomes dark and indistinct with insufficient contrast in the details, which is a serious problem. In order to solve this problem, a low-frequency alternating electric field was applied to the developing gap according to the method disclosed in the above-mentioned Japanese Patent Publication No. Sho 58-32375. This method surely improves the tone reproduction characteristics, and thereby the latent image is developed comparatively faithfully according to the surface potential of the photosensitive body. However, it was also found that this method was effective for pictures or images of 65 line/in. or less line density and not effective for pictures of higher line density.
The ineffectiveness of the application of the low-frequency alternating electric field is reasoned that the indistinctness of the picture of a high-density dot copy is not due to the critical developing characteristics of high .gamma. owing to the flying threshold, but is due to a fact that the electric field corresponding to the electrostatic latent image is not faithful to the electrostatic latent image, and hence image section and the nonimage section on the toner carrying body are not different from each other in electric field, that is, the image section and the nonimage section are not contrasted in respect of electric field.
Furthermore, if the resistance and the thickness of the toner carrying body is not appropriate, for instance, when an ordinary metallic sleeve is employed, any reverse electric field is not produced even in the vicinity (10 to 20 .mu.m) of the photosensitive body. Consequently, the toner flying without discrimination of the image section and the nonimage section gains kinetic energy in the developing gap and does not fly faithfully along the lines of electric force, so that the toner adheres to the nonimage section as well.
In the conventional noncontact transfer developing method, the above-mentioned two problems, i.e., (1) the deficiency in the electric field produced over the toner carrying body according to the electrostatic latent image between the image section and the nonimage section and (2) the deviation of the flying toner from the line of electric force were the causes of the indistinctness of the image section of a high line density and insufficient power of resolution, namely, the causes of deficiency in minute and faithful reproduction.